1. Field of the Invention
The present invention relates to test equipment. More particularly, the invention relates to an apparatus and method for verifying proper operation of a communications bus.
2. Description of Related Art
Over the last few decades, commercial aircraft have become a necessary mode of travel for personal and business reasons. In order to improve passenger comfort, many commercial airlines now offer onboard telephone services as well as in-flight entertainment such as video games, pay-per-view movies, hotel reservation services, and the like. Such amenities require supporting electronics distributed throughout the aircraft. Maintenance of the sophisticated electronics in in-flight entertainment systems (IFES) is a major concern of the airlines.
Airline profitability depends on quick servicing and maintenance of the aircraft. Aircraft servicing typically occurs on the ground, which results in lost flying time and lost profits. Thus, systems which allow quick repairs and facilitate turnaround time are an important part of maintaining and/or increasing airline profitability.
One component which is particularly difficult to troubleshoot is a bus carrying in-flight entertainment data throughout the aircraft. Unlike other components, a bus that carries in-flight entertainment data distributes the data throughout the aircraft, and thus links many different components. Data dropout, noise and bus errors may be introduced at many different points along the bus. The components coupled to the bus may also introduce errors. Determining the source of data errors on a bus can be a time-consuming and tedious procedure. Such troubleshooting consumes service personnel time, and keeps aircraft on the ground.
A second factor which makes troubleshooting difficult is that the IFES system compresses data, such as audio signals using the standard Adaptive Differential Pulse Code Modulation (ADPCM) method. ADPCM compressed data is very sensitive to data errors, as will be explained.
The basic algorithm for the compression of 16-bit linear data to 4-bit ADPCM data and the decompression of 4-bit ADPCM data to 16-bit linear data works as follows. The algorithm finds the difference between the original 16-bit data and the predicted value. Since the difference tends to be a small value, the difference is usually represented by a smaller number of bits. This difference is quantized to a 4-bit compressed pattern using a quantizer step size.
During decompression, the 4-bit compressed pattern is expanded using the same quantization step size to obtain the same linear difference used in compression. To correct for any truncation errors, a binary representation of a value of 0.5 is added during the decompression. This difference is then added to the predicted value to form a prediction for the next sequential 16-bit data. The 4-bit compressed pattern is used to adjust an index into a step size table. This index points to a new step size in the step size table. The index variable and the predicted sample are the two important parameters for decompression.
Since the standard ADPCM algorithm encodes only the difference between consecutive samples, any transmission line error or drop-out of samples will lead to data errors. These data errors are cumulative and are not recoverable. Thus, digital buses are particularly sensitive to drop-outs or transmission line errors.
Thus, a method and device which allows rapid troubleshooting of a bus carrying digital data throughout the aircraft are needed.